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The New Zealand Railways Magazine, Volume 6, Issue 1 (May 1, 1931)

[section]

We now come to the question of surveying as applied to Civil Engineering, a branch of surveying usually demanding extreme accuracy.

In figure No. 13 is shown a survey for a tunnel through an inaccessible mountain range (except by the Pass followed by the survey). Proceeding from the point A, a survey is made to the point B, with great care and checking. In computing this survey it is found that the point B is 35 chains north, and 55 chains east, of the point A. We now have the triangle A B C of which two sides are known, and also the angle at C. The angles at A and B and the length of the side A B can now be computed, as previously explained, giving the length of the straight—included in which is the tunnel and also the bearing of the tunnel in relation to true north. In the diagram the dotted lines are computed and the full lines observed and measured. The grade of the tunnel is fixed by ascertaining the relative height of the points A and B. This will be dealt with when explaining levelling practice. The figures above the line are the measured distances, and below the line the bearing or angle to true north.